A gas turbine engine includes in flow series arrangement a compressor, a combustor, and a turbine. The compressor and the turbine are mounted for rotation on a shaft which drives a gearbox. Coupled to the gearbox are accessories such as an AC electric generator and lube pump.
To start a gas turbine engine, a starter motor applies torque to the engine's shaft. As the shaft starts to rotate, air is inducted into the compressor, compressed and then discharged in the combustor. Concurrently, the engine's fuel control system feeds fuel into the combustor in accordance with a preprogrammed fuel schedule to precisely maintain the proper fuel to air ratio in the combustor. At a rotational speed of about 10 to 20 percent of the engine's operating speed, the conditions in the combustor become such that the fuel/air mixture can be ignited. This condition is generally referred to as light-off. Should the fuel to air ratio be either too rich or too lean, light-off will not occur and the engine will experience a hung start. After light-off the starter motor torque is augmented by torque from the engine's turbine. At about 50 percent of operating speed the starter motor is shut off, the engine is now self sustaining and accelerates itself to operating speed.
DC motors are commonly used as starters. A disadvantage of DC motors is for a given power supply that their torque versus speed characteristic is fixed. Consequently, the DC motor must be sized to produce starting torque under the worst condition, (i.e. greatest engine drag), which is a cold soaked engine. As a result, DC motors are not efficient at warmer temperatures. Another disadvantage to DC motors is that they cannot dwell at a particular speed or in any way change their output as a function of energy consumption.
To overcome the disadvantages associated with DC motors, it has been proposed to use the engine's AC generator as an AC starter motor. Copending U.S. patent application Ser. No. 08/011,022, now U.S. Pat. No. 5,325,042, and Ser. No. 07/987,524, incorporated herein by reference, disclose power converters that converts an AC generator into an AC motor. Generators in combination with such power converters are referred to as starter/generators. Because the power converter can regulate the power consumption of the starter/generator from either a DC or AC source, it can be controlled to provide any starting torque characteristic, (torque vs. speed), desired.
Accordingly, a need exists for a control system for a starter/generator that maintains a desired acceleration of the engine with minimum expenditure of energy, and which automatically adjusts the amount of torque to account for changes in engine drag due to cold soak or any other conditions to which the engine is exposed.